1. Industrial Field of the Invention
The present invention relates to a plastic-molded-type semiconductor device and, more particularly, to a structure in which there is effected a connection between that of a heat conductive member for heat radiation and a plastic encapsulant.
2. Description of the Prior Art
In a semiconductor device, the amount of heat generated from a chip tends to be increased in accordance with a higher level of device integration. In order to ensure the reliability of the device to obtain favorable electro-properties, however, heat generated from the chip must be radiated out of the device effectively to thereby maintain the temperature of the chip at not more than a certain level. However, plastic-molded-type semiconductor devices, which represent the majority of semiconductor devices, are less efficient in heat radiation because the thermal conductivity of resin is much lower than that of a metallic or ceramic material. According to a known and conventional scheme, a semiconductor device is designed to employ a heat conductive member having a high thermal conductivity and which is thermally connected on one side thereof to a chip while the opposing surface of this member is exposed from the surface of the plastic encapsulant. This type of device which enhances the heat radiation efficiency of the plastic-molded-type semiconductor device is disclosed in Japanese Patent Unexamined Publication Nos. 59-28364 and 60-165745.
In each of the above-described conventional examples, the heat conductive member for heat radiation is adhered to the chip and, also, it is exposed out of the plastic encapsulant. However, when the adhesion interfaces between the side surfaces of the heat conductive member and the plastic encapsulant become separated, the moisture in the environment will permeate through the interfaces and reach the chip easily. In case moisture does reach the chips in this manner, the function of the chip will be damaged, for example, fine aluminum wiring on the surface of the chip will be corroded.
In general, the cross-sectional configuration of that portion of the heat conductive member at the portion where the member is covered with the resin, especially at the same level as the surface of the resin, is a polygon such as a square and a rectangle corresponding to the shape of the chip. However, the heat conductive member is usually made of a metallic or ceramic material and, consequently, its linear expansion coefficient is smaller than that of the plastic encapsulant. Such differences in the expansion coefficients leads to a difference between linear expansions of these component parts. Therefore, high shearing stress is generated particularly at corner portions of the adhesion interfaces between the heat conductive member having a polygonal cross-sectional configuration and the plastic encapsulant, which results in a problem that interfacial separation is likely to occur.
Moreover, since the linear expansion coefficient of the heat conductive member is smaller than that of the plastic encapsulant, the resin is constructed by a larger degree when the resin is cooled from the mold temperature to the ordinary temperature, and tightening force from the resin is exerted on the side surfaces of the heat conductive member. Such stress for compressing the adhesion interfaces serves to prevent interfacial separation or generation of a gap.
However, when the polygonal heat conductive member is embedded in the resin, the tightening force from the resin is concentrated in the vicinity of the corner portions of the heat conductive member and hardly exerted on central portions of its sides. Further, even tensile stress develops in some cases. Consequently, the conventional plastic-molded-type semiconductor device including the polygonal heat conductive member involves a problem that it is difficult to obtain adequate plastic molded reliability because separation of the adhesion interfaces between the heat conductive member and the resin is apt to take place.